As the catalysts for producing an olefin polymer such as a homopolymer of ethylene or an α-olefin, and a copolymer of ethylene/an α-olefin, the catalysts containing a titanium compound supported on magnesium halide in the active state have been conventionally known (hereinafter, the term “polymerization” may be described to encompass both of “homopolymerization” and “copolymerization”).
As the catalyst for olefin polymerization, a catalyst containing titanium tetrachloride or titanium trichloride, which is called a Ziegler-Natta catalyst, a catalyst composed of a solid titanium catalyst component comprising magnesium, titanium, halogen and an electron donor, and an organometallic compound, and the like have been widely known.
The latter catalyst exhibits high activity in the polymerization of α-olefins such as propylene and butene-1, in addition to ethylene. Also, the obtained α-olefin polymer may have high stereoregularity.
It is reported that when among these catalysts, in particular, a catalyst comprising a solid titanium catalyst component having an electron donor, supported thereon, selected from carboxylic acid esters, typically exemplified by phthalic acid esters, and as a co-catalyst component, an aluminum-alkyl compound, and a silicon compound having at least one Si—OR (wherein R is a hydrocarbon group) is used, excellent polymerization activity and stereospecificity are exhibited in JP-A No. 57-63310 (Patent Document 1), etc.
The obtained polymers obtained by using the above-described catalyst often have narrower molecular weight distributions, as compared with the polymers obtained by using a Ziegler-Natta catalyst. It is known that the polymers having narrow molecular weight distributions tend to have “low melt flowability”, “low melt tension”, “poor moldability”, “slightly low rigidity”, or the like. On the other hand, from the standpoints of improvement of productivity, reduction in cost, or the like, various high speed molding technologies such as, for example, high speed stretching technologies aimed at improvement of productivity of the stretched film, have evolved.
When the polymers having relatively narrow molecular weight distributions as described above are to be stretched at a high speed, for example, the neck-in or flapping of the films may become more remarkable due to shortage of melt tension, and thus it becomes difficult to improve the productivity. Therefore, the polymers having higher melt tensions are demanded by the market.
In order to solve these problems, there have been many reports on a method for broadening the molecular weight distributions of the polymers by producing the polymers having different molecular weights by means of a multi-stage polymerization (JP-A No. 5-170843 (Patent Document 2)), a catalyst comprising plural kinds of electron donors (JP-A No. 3-7703 (Patent Document 3)), a catalyst using a succinic acid ester having an asymmetric carbon as the electron donor contained in the solid titanium catalyst component (pamphlet of International Publication WO 01/057099 (Patent Document 4), pamphlet of International Publication WO 00/63261 (Patent Document 5), pamphlet of International Publication WO 02/30998 (Patent Document 6)), and the like.
On the other hand, JP-A No. 2001-114811 (Patent Document 7) and JP-A No. 2003-40918 (Patent Document 8) disclose a solid catalyst component for olefin(s) polymerization, which is obtained by bringing into contact with a titanium compound, a magnesium compound and an electron-donating compound, and a catalyst for olefin(s) polymerization comprising the catalyst component. As the electron-donating compound, 1,2-cyclohexanedicarboxylic acid esters having a purity of the trans-isomers of 80% or more are used in the invention as described in Patent Document 7; and cyclohexenedicarboxylic acid diesters are used in the invention as described in Patent Document 8. As a specific example of these cyclohexenedicarboxylic acid diesters, 1-cyclohexenedicarboxylic acid diester only is disclosed, in which the alkoxycarbonyl group is bonded to the 1-position and the 2-position of the cyclohexene ring of 1-cyclohexene (paragraphs [0021] to [0024], and Examples). However, Patent Documents 7 and 8 have no description on the molecular weight distributions of the olefin polymers.
[Patent Document 1] JP-A No. 57-63310
[Patent Document 2] JP-A No. 5-170843
[Patent Document 3] JP-A No. 3-7703
[Patent Document 4] Pamphlet of International Publication WO 01/057099
[Patent Document 5] Pamphlet of International Publication WO 00/63261
[Patent Document 6] Pamphlet of International Publication WO 02/30998
[Patent Document 7] JP-A No. 2001-114811
[Patent Document 8] JP-A No. 2003-40918